1. Field of the Invention
The invention is in the field of sonic transducers as particularly applying to audio speakers.
2. Description of the Prior Art
The terms "sonic" and "sound" are used herein to mean the complete spectrum of compression wave frequencies including audio frequencies and frequencies above and below the audio range.
"Diaphragm-like movement" is defined as the gross flexural warping or bending associated with conventional speaker cones, thin membranes or plates.
Conventional sonic transducers and speaker systems utilize a diaphragm action to serve as an air pump to generate the compressional wave signals in the surrounding medium. Such systems show a high degree of inertial effects and are incapable of reproducing the peaks and sharp spikes which are associated with most sources of sonic energy. The waveforms associated with most sources which generate sonic energy (herinafter sometimes simply referred to as "sonic energy sources"), including but not limited to almost all natural sound sources, musical instruments, voice, sources of mechanical noises such as machinery, percussive or explosive sound sources, and other, consist to a large extent of abrupt amplitude spikes, pulses and other transients having abrupt rise and fall times. Thus, while most present day speaker systems are designed for low inertial impedance, they nevertheless are nonresponsive to short pulse durations, and are therefore inherently incapable of accurately reproducing the sounds generated by musical instruments, the human voice, and most other sonic energy sources. Conventional speaker systems even fail to accurately reproduce sine waves, since they flatten them out and thereby introduce distortions into them. Although many attempts have been made to reduce the inertia of typical diaphragm-type speakers, basic nonlinearity problems nevertheless exist and the diaphragm is inherently limited by its mechanical pistonlike action which serves as an air pump.
Piezoelectric crystals have been utilized both as air pumps per se and to drive diaphragms and produce flexural deformations in metallic air driving means such as shown in Spitzer et al U.S. Pat. No. 2,911,484, Ashworth U.S. Pat. No. 3,366,748, Watters et al U.S. Pat. No. 3,347,335 and Kompanek U.S. Pat. No. 3,423,543. These prior art teachings are designed to produce a flexing or mechanical deformation of the diaphragm or air driving member. Consequently, every effort has been made to support the air driving or diaphragm member with a minimum of friction and in an undamped structure. Such an arrangement is relatively inefficient and inherently incapable of reproducing fast rise time and fast fall time pulses.
Present day speaker arrangements usually require at least three separate speakers to reproduce the full range of audio frequencies. These speakers, the woofer, mid-range and tweeter are connected to the audio amplifier output by sophisticated crossover networks so as to feed each speaker only those portiions of the frequency range which it is best able to reproduce. The relatively large inertia of the woofer makes it incapable of producing the high frequencies while the tweeter has small cone excursions suitable for high frequency reproduction but not low frequency reproduction. Even utilizing the crossover networks, however, tweeter designs are not capable of responding to the sharp spikes or high nearly instantaneous peaks associated with most sonic energy sources. Thus, while tweeters may be rated to respond to 20KH.sub.z or more, this rating is relative to a sine wave input signal which is characteristic of an excited speaker cone; the weight or inertia of a diaphragm-like one is incapable of responding to the abrupt amplitude rise and fall times of most sonic energy sources, even though the sharp amplitude signals may exist on the tape or other program source. The inertial effect is a fundamental shortcoming of all diaphragm-type speakers.
The best tweeters available today are rated as being responsive to sine wave signals up to 25KH.sub.z. However, according to the accepted definition of square wave response a minimum of at least 10 octaves (of a sine wave) are necessary to approach a square wave. Thus, under this square wave definition, even the best tweeters only have a square wave response capability of one-tenth of 25KH.sub.z, or 2.5KH.sub.z, which is totally inadequate for responding to a large portion of the sonic energy content of most sonic energy sources.
New methods of deriving signals which eliminate the inertial effects of conventional microphones have particularly emphasized the serious inertial effect deficiencies of conventional speaker systems. For example, recordings can now be made with modern non-inertial type pick-ups, so that the recordings contain an electrical representation of sonic information that is far more accurate and complete than conventional speaker systems are capable of reproducing. As another example, piano sounds picked up by modern non-inertial pick-up systems become "cracked" or "break up" at predictable points when played through all conventional tweeters.
A further problem with conventional speakers is that the paper of conventional speaker cones inevitably introduces paper-like sounds into the speaker output, and even the metal diaphragm of a tweeter horn injects metal-like noises into the output. Such undesirable noises cannot be damped, since the speaker output depends upon the vibratory pumping action of such elements.
Diaphragm-like speakers also inherently produce a highly directional sound pattern which becomes more constricted with higher frequencies, and in the case of the high frequencies associated with tweeters takes the form of a narrow pencil-like radiation beam. The directional aspects of the diaphragm speakers makes their relative position and orientation an important and often expensive consideration in designing sophisticated audio speaker systems.